Differential output inductor for class D amplifier

ABSTRACT

A circuit includes a first input terminal for receiving a first pulsed voltage and a second input terminal for receiving a second pulsed voltage. The circuit further includes a load and an LC filter. The LC filter includes a coupled inductor pair that includes a first winding and a second winding magnetically coupled to each other. The first winding is coupled between the first input terminal and the load, and the second winding is coupled between the second input terminal and the load. A frequency of a first current flowing through the first winding is increased by the second pulsed voltage applied to the second winding.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. application Ser.No. 13/344,392, filed Jan. 5, 2012, which claims the benefit of U.S.Provisional Application No. 61/429,974 filed on Jan. 5, 2011, and whichis a Continuation-In-Part of U.S. application Ser. No. 12/605,311, filedon Oct. 23, 2009 (now U.S. Pat. No. 8,115,366), which claims the benefitof U.S. Provisional Application No. 61/107,982, filed on Oct. 23, 2008,and U.S. Provisional Application No. 61/182,325, filed on May 29, 2009,which are incorporated by reference for all purposes as if fully setforth herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

1. Field

This invention relates generally to audio applications and ultrasonictransducers, and more particularly, to a system and method for drivingaudio speakers and ultrasonic transducers.

2. Description of the Related Art

Ultrasonic transducers have been in use for many years. During that timelittle change has occurred in the way they are driven. Current drivingcircuits are based on resonant technology that has many limitations.

Current technology depends on resonant circuits to drive ultrasonictransducers. Resonant circuits are, by definition, designed to operatein a very narrow range of frequencies. Because of this the transducertolerances are held very tightly to be able to operate with the drivingcircuitry. In addition, there is no possibility of using the samedriving circuit for transducers with different frequencies, and thecircuit must be changed for every transducer frequency.

To drive ultrasonic transducers, a method is often required to generatefrequencies with high accuracy and very high frequency shifting speed.Tank circuits have been used to address this need. Tank circuits, whichcomprise a particular transducer coupled to circuitry uniquelyconfigured to work with the transducer, allow the transducer to bedriven at the resonance frequency specific to the particular transducer.A draw back with prior art systems and methods is that the circuitry ofthe tank circuit often cannot be used with another transducer having adifferent resonance frequency.

There is a need for a system and method for driving any transducerregardless of the resonance frequency of the transducer. Such a systemand method may drive multiple transducers each having a differentfrequency, thereby allowing device manufacturers to take advantage ofeconomies of scale by implementing the same driver with varioustransducers having different frequencies.

SUMMARY

Briefly and in general terms, the present invention is directed to asystem and method for driving ultrasonic transducers.

In aspects of the invention, a circuit includes a first input terminalfor receiving a first pulsed voltage, and a second input terminal forreceiving a second pulsed voltage. The circuit further includes a loadand an LC filter. The LC filter includes a coupled inductor pair thatincludes a first winding and a second winding magnetically coupled toeach other. The first winding is coupled between the first inputterminal and the load, and the second winding is coupled between thesecond input terminal and the load. A frequency of a first currentflowing through the first winding is increased by the second pulsedvoltage applied to the second winding.

In aspects of the present invention, a method of filtering a signalincludes applying a first pulsed voltage to a first winding. A secondpulsed voltage is applied to a second winding, wherein the first andsecond windings are magnetically coupled. An output current is providedto a load coupled between the first and second windings. The outputcurrent is sourced from the first winding and sunk at the secondwinding. A frequency of a first current flowing through the firstwinding is increased by the second pulsed voltage applied to the secondwinding.

The features and advantages of the invention will be more readilyunderstood from the following detailed description which should be readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For proper understanding of the invention, reference should be made tothe accompanying drawings, wherein:

FIG. 1 is a schematic diagram showing a circuit configured to determineadmittance according to some embodiments of the invention.

FIG. 2 is a schematic diagram showing a circuit having an exclusive ORgate, the circuit configured to determine a phase angle according tosome embodiments of the invention.

FIG. 2a is a flow diagram showing waveforms into and out of an exclusiveOR gate of the circuit of FIG. 2.

FIG. 3 is a block diagram showing a system for driving a transduceraccording to some embodiments of the invention.

FIG. 4 is a flow diagram showing elements of a frequency controlleraccording to some embodiments of the invention.

FIG. 5 is a block diagram showing a frequency tracker utilizingadmittance according to some embodiments of the invention.

FIG. 6 is a block diagram showing a frequency tracker applying phaseerror to a PD controller according to some embodiments of the invention.

FIG. 7 is a block diagram showing a current controller applying currenterror to a PID controller according to some embodiments of theinvention.

FIG. 8 is a block diagram showing an output filter for filtering a drivesignal to a transducer according to some embodiments of the invention.

FIG. 9 is a schematic diagram showing a prior art output filtercomprising a cascaded LC filter.

FIG. 10 is a schematic diagram showing an output filter comprising acoupled LCLC filter having magnetically coupled inductors according tosome embodiments of the invention.

FIG. 11 is a chart showing PWM signals for a dual channel D classamplifier with differential outputs in which the switching periods forall the signals are aligned.

FIG. 12 is a chart showing PWM signals for a dual channel D classamplifier with differential outputs in which a phase shift is insertedbetween PWM signals for the two channels.

FIG. 13 is a schematic diagram showing a prior art multiphase buckconverter with coupled inductors.

FIG. 14 is a schematic diagram showing a differential amplifier outputstage with coupled inductors according to some embodiments of theinvention.

FIG. 15 is schematic diagram showing a simplified general model of thecoupled inductor of FIG. 14.

FIG. 16 is a chart showing waveforms for FIG. 14 when inductors are notmagnetically coupled.

FIG. 17 is a chart showing waveforms for FIG. 14 when inductors aremagnetically coupled, the solid lines for inductor current correspondingto inductors magnetically coupled and broken lines for inductor currentcorresponding to inductors without magnetic coupling.

FIG. 18 is a chart showing waveforms for a 20 kHz output signal with 90μH/94 nF filters with added 180 phase shift in a second oscillator,Vdc=100 V, Rload=100, the solid lines for inductor current correspondingto inductors magnetically coupled and broken lines for inductor currentcorresponding to inductors without magnetic coupling.

FIG. 19 is a diagram showing a D class amplifier with differentialoutputs in which a first PWM output signal is delayed to generate asecond PWM output signal according to some embodiments of the invention.

FIGS. 20, 21 and 22 show simplified diagrams showing varying arrangesfor a transformer with leakage, the transformer corresponding tomagnetically coupled inductors in an output filter according to someembodiments of the invention.

FIGS. 23A, 23B, 23C, 23D, 23E, 23F and 23G illustrate various circuitscontaining various combinations of a D-inductor and filter componentsaccording to various embodiments of the invention.

FIG. 24A illustrates a class D amplifier according to some embodimentsof the invention.

FIG. 24B illustrates a circuit for generating control signals for theclass D amplifier shown in FIG. 24A according to some embodiments of theinvention.

FIG. 24C illustrates various input and output signals of the circuitshown in FIG. 24B according to some embodiments of the invention.

FIG. 24D illustrates another circuit for generating the control signalsfor the class D amplifier shown in FIG. 24A according to someembodiments of the invention.

FIGS. 25A and 25B illustrate various voltage and current in a class Damplifier with an uncoupled design.

FIGS. 26A and 26B illustrate various voltages and current in a class Damplifier with a D-inductor design according to some embodiments of theinvention.

FIG. 27 illustrates normalized inductor current ripple measured in anamplifier with the uncoupled design and the D-inductor design.

FIGS. 28A and 28B illustrate output waveforms measured in an amplifierat near full output power for the uncoupled design and the D-inductordesign.

FIG. 29 illustrates normalized inductor current rippled in an amplifierwith the uncoupled design and the D-inductor design.

FIGS. 30A and 30B illustrate various inductor current ripple measured inan amplifier with the uncoupled design and the D-inductor design.

FIGS. 31A and 31B illustrate efficiency measured in an amplifier withthe uncoupled design and the D-inductor design.

FIGS. 32A and 32B illustrate power loss measured in an amplifier withthe uncoupled design and the D-inductor design.

FIGS. 33A, 33B and 33C illustrate a construction, operation and currentripple of a class D amplifier with the D-inductor design according tosome embodiments of the invention.

FIGS. 34A and 34B illustrate a single phase class D amplifier and amulti-phased class D amplifier according to some embodiments of theinvention.

FIGS. 35A and 35B illustrate a construction and operation of amulti-phased class D amplifier according to some embodiments of theinvention.

FIG. 36 illustrates an implementation of the multi-phased class Damplifier of FIG. 35A according to some embodiments of the invention.

FIGS. 37A and 37B illustrate current ripple waveforms and total currentripple waveforms in various class D amplifier designs.

FIGS. 38A, 38B, 38C and 38D illustrate simulation results of variousclass D amplifier designs.

FIGS. 39A, 39B and 39C illustrate simulation results of various class Damplifier designs.

FIGS. 40A, 40B and 40C illustrate simulation results of various class Damplifier designs.

FIGS. 41A, 41B and 41C illustrate total output current ripple in variousclass D amplifier designs.

FIGS. 42A, 42B, 43A, 43B, 43C, 43D, 44A, 44B, 45A, 45B, 46A, 46B, and 47illustrate various implementations of the D-inductor design according tovarious embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Some embodiments of the present invention involve hardware and software.The hardware may include a switching amplifier to create a sine waveoutput to an ultrasonic transducer. The ultrasonic transducer can be apiezoelectric transducer. The switching amplifier can be run with highefficiency over a broad range of frequencies and can, therefore, be usedto drive transducers of many frequencies. The switching amplifier canalso drive transducers that do not have tightly held frequencytolerances thereby reducing transducer production cost. This allows forreduction of production cost due to economies of scale and allows forcustomers that use different frequency transducers to always be able touse the same driver.

Previous ultrasonic generators have relied on resonant power sources oranalog amplifiers to drive the transducer. In some embodiments of thepresent invention a class D or class E amplifier is used to amplify theoutput of a digitally controlled AC source. This technique frees themanufacturer and user from the requirement of designing a resonantsystem around a specific transducer. Instead, this system is usable forany transducer over a broad range of frequencies.

Previous class D and class E amplifiers have used traditional LC orcascaded LC filters to significantly reduce the effects of the class Dor E carrier frequency on the signal frequency. In some embodiments ofthe present invention a two phase output signal is used in conjunctionwith a coupled transformer to reduce the effect of the carrier frequencyto several times lower than could be done with similar size and costcomponents with the traditional LC type filters.

In some embodiments of the present invention, software could runentirely on low cost, 16-bit, integer-only microcontrollers. The morepowerful DSP (digital signal processor) modules typically required inprior art are not required in the present invention, although DSPmodules could be used in some embodiments.

A method is required to generate a wide range of frequencies with highaccuracy and very high frequency shifting speed. A digital synthesizercould be used in an ultrasonic system to allow rapid and flexiblefrequency control for output of a frequency generator.

In some embodiments, dead time is minimized in switching circuits inorder to minimize the output impedance to the transducer. The phrase“dead time” is the time in power switching circuits when all switchingelements are off to prevent cross conduction. When determining theresonant frequency a minimum or maximum admittance is used. Theadmittance measured will vary much less between in resonance and out ofresonance in a low Q system than in a high Q system. The dimensionlessparameter “Q” refers to what is commonly referred to in engineering asthe “Q factor” or “quality factor.” Because Q is directly affected bythe impedance of the driving circuit, this impedance must be kept verylow. In addition to the commonly considered impedances of the outputtransformer, driving semiconductors, PCB (printed circuit board) andother directly measurable impedances, Applicants have found that thedead time has a very strong effect on the output impedance of thedriver. As such, the switching circuit is configured to have a verysmall (approximately 50 nanoseconds) dead time. In some embodiments, theswitching circuit has a dead time that is greater than or less than 50nanoseconds.

For optimum operation, it is critical that the transducer be run at ornear its resonant frequency point. The resonant frequency point of thetransducer is defined as the frequency at which maximum real power istransferred from the drive amplifier to the transducer. Much work hasbeen done to determine the best method for measuring when a transduceris at or near resonance.

Applicants have found that the admittance of the transducer gives areliable indication of the proximity of the transducer to its resonantfrequency point. Admittance is defined as the RMS (root-mean-square)amplitude of the transducer drive current divided by the RSM amplitudeof the transducer drive voltage. The circuit 10 shown in FIG. 1determines the RMS (root mean square) value of the admittance 12 of adriven transducer in real time. The RMS value of the admittance is usedfor analysis by software contained and run by the hardware. The RMSvalue of the admittance 12 is obtained from the RMS voltage 14 acrossthe transducer and RMS current 16 supplied to the transducer.

The circuit in FIG. 1 is an example of a circuit that measures thereal-time admittance of the load. RMS voltage 14 and RMS Current 15 arefiltered. The filtered signals for voltage 16 and current 17 are fedinto an analog divider 18 and the resultant output 19 is fed to an RMSconverter. The final output 20 is RMS admittance. This is a known meansto measure admittance.

Applicants have found that the phase of the transducer also gives areliable indication of the proximity of the transducer to its resonantfrequency point. Phase is defined as the phase angle between thetransducer drive voltage and transducer drive current.

The circuit shown in FIG. 2 is an example of a circuit that derives thephase relationship of two input signals. The voltage driving signal fromthe generator 55 is buffered and filtered by amplifier 57. The currentof the generator signal is found by passing the generator output throughcurrent transformer 57 and then buffering and filtering this signalthrough amplifier 59. Each output (current and voltage) is put into acomparator. The output of the comparator will be high when therespective signal is above zero volts and will be low when it is belowzero volts. The output of the comparators, therefore, transition whenthe input signal crosses zero. If the point where each signal crosseszero is compared an indication of the phase relationship will be known.To find this phase relationship and convert it into an analog voltage,an exclusive OR gate 62 is used and is output is passed through a simpleRC filter. The waveforms into and out of the exclusive OR gate are shownin FIG. 2a . In this example signal 63 represents the output of thecomparator for the voltage and signal 64 represents the output of thecomparator for the current signal. The reader can observe that the twosignals are out of phase and that the phase relationship changes at time66. Persons skilled in the art will recognize that the output of anexclusive OR gate will be high when the input signals are different andlow when they are the same. Signal 65, therefore, shows the output ofthe exclusive OR gate. The RC filter effectively integrates the waveform65 resulting in signal 67. As can be seen, the result is an analogvoltage 67 that is proportional to the phase relationship of the twoinput waveforms, 63, 64. This analog signal 67 is then input to theprocessor.

FIG. 3 depicts a system and method of driving an ultrasonic transducer.The method may be implemented by hardware and software combined toprovide adaptive feedback control to maintain optimum conversion ofelectrical energy provided to the transducer to motion of transducerelements.

In FIG. 3, the system 200 includes two controllers: a current controller202 that maintains a constant commanded transducer current; and afrequency controller 206 that searches for and tracks the operatingfrequency. A controller scheduler 204 interleaves the operation of thetwo controllers 202, 206 to reduce the operation of one controlleradversely affecting the operation of the other controller.

The drive 208 provides a drive signal of controlled voltage andcontrolled frequency to the transducer 210. An output parameter sensecircuit 212 senses transducer drive voltage and transducer drive currentand generates a measure of current 218, admittance 220, and a frequencycontrol parameter 222. The frequency control parameter is different indifferent embodiments.

Current 218 is applied as an input to the current controller 202 whichgenerates a voltage 214 applied to the drive 208. The current controller202 sets the voltage 214 to maintain the current required for correctoperation of the transducer 210 in its given application.

The frequency controller 206 performs two functions: frequency scanningand frequency tracking. The frequency scanning function searches for afrequency that is at or near the resonant frequency of the transducer.The frequency tracking function maintains the operating frequency at ornear the resonant frequency of the transducer.

When the frequency controller 206 is frequency scanning, admittance 220is applied to it as an input. The frequency controller sweeps the drivefrequency over a range of frequencies appropriate for the transducer andapplication, searching for the resonant frequency.

When the frequency controller 206 is frequency tracking, a frequencycontrol parameter 222 is applied to it as an input. The frequencycontroller sets the frequency required for correct operation of thetransducer in its given applications.

When the frequency controller 206 performs either frequency scanning orfrequency tracking, it applies the calculated frequency 216 to the drive208.

The drive 208 may include the switching amplifier and switching circuitsdescribed above. The frequency controller 206 may include the digitalsynthesizer described above.

Frequency Controller

As previously mentioned, the frequency controller 206 performs twofunctions: frequency scanning and frequency tracking.

In many applications, initial application of drive to the transducer atits resonant frequency is critical. When, due to variations intransducer characteristics, applied power levels, and the mechanicalload the transducer connects to, the resonant frequency is not a prioriknown, the frequency controller may perform a frequency scan toestablish the drive frequency at or near the resonant frequency.

When performing a frequency scan, the frequency controller searches apredefined range of frequencies for the frequency at which thetransducer admittance is maximum. As shown in FIG. 4, the frequencyscanner 300 is made up of three sweep scans: a wide scan 302, which isfollowed immediately by a medium scan 304, which is followed immediatelyby a narrow scan 306. The wide scan includes a ±1 kHz sweep about apredefined frequency, in 4 Hz steps, with a 10 msec settling time aftereach step, and detecting the admittance after each settling time. Themedium scan includes a ±100 Hz sweep about the frequency of maximumadmittance detected by the wide scan, in 2 Hz steps, with a 25 msecsettling time after each step, and detecting the admittance after eachsettling time. The narrow scan includes a ±10 Hz sweep about thefrequency of maximum admittance detected by the medium scan, in 1 Hzsteps, with a 50 msec settling time after each step.

In some embodiments, admittance is detected after each narrow scansettling time and, at completion of the narrow scan, the drive frequencyis set to the frequency of maximum detected admittance.

In some embodiments, phase is detected after each narrow scan settlingtime and, at completion of the narrow scan, the drive frequency is setto the frequency with detected phase closest to the phase required forcorrect operation of the transducer in its given application.

An ultrasonic transducer will often have multiple frequencies at whichthe commanded phase is measured. The frequency of maximum admittancewill always be at or close to the resonant frequency, the frequency ofmaximum real power transfer. For this reason, maximum admittance is usedfor wide and medium scans for the operating point, regardless of themethod used in the narrow scan.

The frequency scanner 300 can be executed at either full power (asdefined by the user) or at a predefined low power of less than 5 watts,measured at transducer resonance.

The frequency controller 206 may optionally perform a fast scan 308 aspart of its operation, immediately prior to initiation of a frequencytrack algorithm. The fast scan includes a ±10 Hz sweep about the currentfrequency, in 2 Hz steps, with a 10 msec settling time after each step.

In some embodiments, admittance is detected after each fast scansettling time and, at completion of the fast scan, the drive frequencyis set to the frequency of maximum detected admittance.

In some embodiments, phase is detected after each fast scan settlingtime and, at completion of the fast scan, the drive frequency is set tothe frequency with detected phase closest to the phase required forcorrect operation of the transducer in its given application. The fastscan 308 can be executed at either full power or at less than 5 wattspower.

The transducer resonant frequency may fluctuate during normal operation.This fluctuation may occur due to changes in operating conditions of thetransducer, such as changes in temperature of the transducer andmechanical load on the transducer. Frequency tracking can be performedto compensate for this fluctuation in resonant frequency.

FIG. 5 shows an embodiment of a frequency tracker. The frequency tracker400 is comprised of two components: a peak detector 402 and a frequencystepper 404. The peak detector samples the transducer admittance 422.The peak detector then commands the frequency stepper 404 to take arandom-size step, between 1 and 10 Hz in a random direction, either upor down. The frequency stepper calculates the random step size anddirection and sends the frequency step, .DELTA. frequency 418, to thefrequency generator 406 which generates the new drive frequency 420 andapplies it to the drive 408 (208 in FIG. 3). The frequency trackerdelays a short time period based on the size of the frequency step(nominally 10 to 50 msecs) to allow the transducer to settle on thenewly commanded frequency. Transducer 410 drive current and transducerdrive voltage are continually monitored and converted to their RMSequivalent values by RMS converters 412 and 414, respectively. Thedivider 416 divides RMS current by RMS voltage to calculate admittance422 which is applied to the peak detector 402. With this admittance, thepeak detector calculates the change in detected admittance that resultedfrom the step in frequency.

If the detected admittance has increased by greater than a predefinedamount, the next step 418 is taken in the same direction as the previousstep, with step size based on the magnitude of the increase inadmittance. For example, the magnitude of the step can be proportionalto the detected increase in admittance. If the detected admittance hasdecreased by greater than a predefined amount, the next step 418 istaken in the opposite direction, with the magnitude of the step beingbased on the magnitude of the increase in admittance. If the detectedadmittance has neither increased by greater than a predefined amount nordecreased by greater than a predefined amount, the admittance is assumedto be at its peak and a zero magnitude “step” is taken. The frequencytracker delays a short time period to allow the transducer to settle andthe peak detection and step sequence is repeated.

The maximum admittance of a transducer may increase, remain unchanged,or decrease, depending on changes in operating conditions of thetransducer. Frequency tracking for increasing and unchanging maximumadmittance values is performed by the above-described frequency trackingmethod. Tracking the resonant frequency associated with a decreasingadmittance maximum is performed by stepping quickly in equal magnitudesteps in both directions about the current frequency until the decreasein admittance stops and increased admittance values are again detected.The Frequency Controller then changes the frequency to again lock on thepoint of maximum admittance.

The frequency tracking method described above can be implemented with analgorithm within software being run by the hardware of the system 200.

Another embodiment of the frequency tracker, shown in FIG. 6, uses thephase angle 516 between the transducer drive voltage and the transducerdrive current to maintain the resonant frequency. For some ultrasonictransducers, the resonant frequency occurs at zero phase. For sometransducers, and related to the transducer operating conditions, theresonant frequency occurs with a negative phase value. Commanded phase518 is empirically selected for a given transducer with given set ofoperating conditions.

The frequency tracker 500 performs frequency tracking by applying aphase angle error term 520 to a Proportional-Derivative (PD) controller502 at regular sampling intervals of between 5 and 20 msecs. The phaseangle error term is calculated to be the difference between the phasetrack command 518 and the measured transducer phase 516. The PDcontroller 502 includes a differentiator, .delta. 502 a, a proportionalgain, KFP 502 b, a differential gain, KFD 502 c, and an output gain, KFO502 d. The output from the PD controller 502 in response to a phaseerror 520 is a step in frequency, A frequency 512, of magnitude and signnecessary to drive the phase error 520 toward zero. The step infrequency 512 is applied to the frequency generator 504 which calculatesthe new frequency 514. The driver drives the transducer 508 at thefrequency 514 from the frequency generator 504.

Current Controller

FIG. 7 shows an embodiment of the current controller 202 in FIG. 3. Thecurrent controller 600 maintains current through the transducer at aconstant, user-commanded level 614. The user commanded level 614 maycorrespond to a desired level of operation of a device containing atransducer. For example, the user commanded level may correspond to adesired energy level of a surgical cutting device containing apiezoelectric transducer.

The current controller 600 varies the current through the transducer byvarying the drive voltage applied across the transducer. Increasing thedrive voltage increases the transducer current and decreasing the drivevoltage decreases the transducer current. In some embodiments, thecurrent controller 600 provides a voltage 610 to the drive 604, and thisvoltage is provided by the drive 604 to the transducer 606.

At a regular sampling intervals, ranging between 5 and 20 msecs, thecurrent controller 600 samples the transducer current and converts it toan RMS current value 612 by an RMS converter 608. At each samplinginterval the current controller 600 calculates a current error term 616by subtracting the sample of the output RMS current 612 from thecommanded current 614.

The current controller 600 applies a current error term 616 to aProportional-Integral-Derivative (PID) controller 602, which generates aresponse 610 to the error 616. The error 616 is integrated by anintegrator 602 a and differentiated by a differentiator 602 b. The error616 and its integral and differential are multiplied respectively by theP, I, and D gains, 602 c, 602 d, 602 e internal to the PID controller,summed, and their sum multiplied by the controller output impedancefactor KCO 602 f to form the controller output voltage 610. Controllergains, 602 c, 602 d, 602 e, 602 f are set to achieve maximum rise timewith an approximately 10% overshoot in the output response to a step inthe input. The output impedance factor 602 f provides both scaling andtranslation from current to voltage. The controller output voltage 610is applied to driver 604 to be amplified to become the transducer drivevoltage.

In some embodiments, the current controller 600 employs two outputimpedance factors 602 f. A larger output impedance factor may be usedfor the first period of time (nominally 500 msecs) to assure thetransducer reaches its steady-state behavior at the given drive power,physical load, and temperature as rapidly as possible. A smaller outputimpedance factor may be used once the transducer has reached itssteady-state behavior. When the switch from the first to the secondoutput impedance factor occurs, the integral of the current errormaintained by the PID controller is modified to prohibit an undesiredtransient in the transducer drive voltage.

In FIG. 3, when the frequency controller 206 sets a drive frequency thatresults in a change in the frequency control parameter 222, because thetransducer current will also change, the current controller 202 willattempt to counter this change. If the frequency controller and thecurrent controller are allowed to operate concurrently, the operation ofthe frequency controller and the current controller may be in conflict.If the effect of the frequency controller 206 is stronger, frequencytracking will take precedence over a constant output current, and theoutput current may wander from the commanded value. Conversely, if theeffect of the current controller 206 is stronger, a constant outputcurrent will take precedence over frequency tracking, and the drivefrequency may wander from the transducer resonant frequency.

To achieve balanced operation, the controller scheduler 204 interleavesthe operation of the frequency controller 206 and the current controller202.

When the frequency controller is performing a scan or search operation,the controller scheduler disables the current controller.

When the frequency controller is tracking frequency, in some embodimentsthe controller scheduler alternates the operation of the twocontrollers. That is, a controller will execute every 5 N msecs, withthe current controller executing for odd N and the frequency controllerexecuting for even N.

In some embodiments, both controllers are allowed to operatesimultaneously, except immediately after a frequency step. When thefrequency controller is tracking frequency, the controller schedulerdisables the current controller for the first M 5-msec periods after afrequency step. The number of periods, M, is typically 2, but can bemore or less than 2. At the end of the M periods, the frequency controlparameter is now only a result of the step in frequency and not ofcontrol exerted by the current controller. The frequency controlparameter is sampled at this time and stored for the next frequencycontroller calculation, and the controller scheduler re-enables thecurrent controller.

Output Amplifier and Filtering

The output of the processor running the code discussed previously is asmall signal with all the characteristics of necessary to drive andultrasonic transducer except for the amplitude. The drive circuit 208,408, 506 can be broken down into two sections as shown in FIG. 8. InFIG. 8 the drive section 71 comprises an amplifier of Class D or E andan output filter.

Prior art has used linear amplifiers for this drive section. These havethe disadvantages of being large, inefficient and costly. Theillustrated embodiment of FIG. 8 uses a switching amplifier which insome cases can be of Class D or E. Use of switching amplifiers is commonin audio applications but new to the field of ultrasonic.

In some embodiments, the drive 208, 408, 506 includes filter circuitry.In some embodiments with a transducer operational range of 20 kHz to 60kHz, the filter circuitry is configured to have a corner frequencyhigher than 60 kHz to avoid excessive resonant peaking Depending on thetype of transducer and its intended use, it will be appreciated that thetransducer operational range can be lower than 20 kHz and/or higher than60 kHz, and the filter circuitry can be configured to have a cornerfrequency higher than the transducer operational range. The carrierfrequency used can be about 10 times that of the transducer resonancefrequency.

In some embodiments the filter circuitry is configured to reducetransmission of the carrier frequency (Fs) from a switching amplifier ofthe drive 208, 408, 506. Non-limiting examples of filter circuitry aredescribed below.

In previous art, the output filter of a switching amplifier is typicallyimplemented with an LC or cascaded LC filter. An example of a cascadedLC filter is shown in FIG. 9. FIG. 9 shows the required elements (L1,C1, L2, C2, L3, C3, L4, C4) and the load (RLOAD).

Part of this invention is a new form of output filter (applicable forany switching amplifier) that includes a coupled inductor as part of theoutput filter. An example schematic of this new coupled LCLC filter isshown in FIG. 10. FIG. 10 shows the required elements (L1-L3, C1, C3,L2, C2, L4, C4) and the load (RLOAD). The coupled inductor is designedto have a relatively large leakage inductance. Leakage inductance isdefined as the residual inductance measured in the winding of atransformer (or coupled inductor) when the unmeasured winding isshorted. When a winding is shorted the magnetizing inductance associatedwith two windings is eliminated and the remaining inductance is seriesconnection of the leakage inductances in both windings. In case ofsymmetrical design for both windings, the leakage inductances are closein value, and can be found by measurement by dividing the measured totalleakage by two. This leakage inductance acts in place of the separateinductors L1 and L3 shown in FIG. 9, in fact, insuring the sameinductance values would insure the same frequency response of thesystem: with separate or magnetically coupled inductors. In addition tothe leakage inductance of the coupled inductor a portion of the signalfrom one winding is coupled to the other winding.

To take advantage of the coupled inductor, a second change is made tothe system. The class D or E amplifier from FIG. 8 is often dual channelamplifier, delivering differential output to the load. As typically thesame signal is amplified for a singe output, one PWM modulator is usedto derive pulses for the both amplifier channels, insuring suchconnection that output of one channel increases voltage, when anotherchannel decrease the output voltage, and vise versa. This is a commonscheme for providing a differential output for such amplifiers. It isalso simple to use the same PWM signal and its inverted signal to driveswitching devices in both channels of the amplifier, as for exampleillustrated in FIG. 11 the switching periods for all the signals arealigned. The proposed scheme, on the other hand, inserts a phase shiftbetween PWM signals for the two channels, as shown in FIG. 12. Theproposed phase shift between periodic signals is 180 degrees, or halfthe period. Phase shift between the signals is shown as Ts/2, half ofthe switching period Ts.

The described phase shift between two or more channels can be found inprior art, for example in multiphase buck converter applications, or inU.S. Pat. No. 6,362,986 to Shultz et al., entitled “Voltage converterwith coupled inductive windings, and associated methods.” U.S. Pat. No.6,362,986 represents closer prior art, as it has phase shift togetherwith magnetic coupling between inductors, as illustrated in FIG. 13,where only two phases of multiphase buck converter are shown. Thisinventions proposed arrangement is shown in FIG. 14, so the differencesfrom prior art in FIG. 13 are illustrated clearly.

Notice that the output voltage of circuit in FIG. 14 is differential,while in FIG. 13 it is not. With zero input signal for the amplifier,the duty cycle of both PWM1 and PWM2 in FIG. 14 is 0.5, soVo1=Vo2=Vdc/2. This relates to zero differential output voltage. Wheninput signal is applied to modulators, if Vo1 rises to positive rail Vdcfrom Vdc/2—then Vo2 is dropping towards zero from the same Vdc/2. Thecurrents in inductors in FIG. 14 are also opposite, as compared to addedcurrents in FIG. 13. If current IL1 is positive (sourcing), then thecurrent IL2 is negative (sinking). Notice also that the average valuesof the IL1 and IL2 in FIG. 14 are absolutely equal, as these outputs areeffectively shorted to each other through the load in series. Themagnetic coupling of proposed arrangement in FIG. 14 is also in phase,relatively to the pins connected to the outputs of the amplifierchannels or phases. The prior art arrangement in FIG. 13 uses inversemagnetic coupling, relatively to the outputs of the buck converterstages. The load in FIG. 13 is typically connected from the commonconnection of all inductors to the ground or return, while the load forcircuit in FIG. 14 should be connected between two differential outputs.

Magnetic coupling between windings in FIG. 14 effectively doubles thefrequency of the current ripple in each winding because when one windingor channel switches it induces a current ripple in the opposite windingeven though that winding did not switch yet (due to the phase shift).

The coupled inductor from FIG. 14 can be modeled as ideal transformer T1in FIG. 15, with ideal magnetic coupling, with added magnetizinginductance Lm and leakages in each winding Lk1 and Lk2. These leakageinductances could be also made external, for example, standardtransformer with good magnetic coupling and negligible leakage could beused with external separate inductance added in series with eachwinding. The general coupled inductor model for arrangement in FIG. 14is shown in FIG. 15, where Lk1 and Lk2 can be leakage inductances of thecommon structure, or dedicated external inductors.

Waveforms for the circuit in FIG. 14 with no magnetic coupling betweeninductors is shown in FIG. 16. Inductors work as energy storagecomponents, ramping current up and down under applied voltage across therelated inductor. Applied voltage changes only due to the switching ofthe related power circuit, where the inductor is connected. FIG. 17shows the same waveforms but when inductors in FIG. 14 are magneticallycoupled. Due to magnetic coupling, applied voltage across the leakageinductances is changed not only due to the switching of the relatedpower circuit, where the inductor is connected, but also when anotherpower circuit switches. This effectively doubles the frequency of thecurrent ripple in each coupled inductor, for the illustrated case wheretwo inductors are magnetically coupled, and the phase shift between twodriving signals is 180 degrees. This coupling effect leads to thedecrease of the current ripple amplitude in the each inductor. FIG. 18illustrates the decrease of the current ripple in inductor forparticular example. Sine wave signal of the 20 KHz frequency isdelivered at the differential output of the amplifier, where twochannels have a phase shift for the switching signals of 200 KHz mainPWM frequency. The bottom traces show inductor current without and withmagnetic coupling, clearly indicating the current ripple decrease.

The decreased current ripple offers several benefits to the circuit andits performance. Decreased current ripple makes it easier for the outputfilter to achieve low noise levels and low output voltage ripple at theoutput, in other words—either smaller attenuation could be used ascompared to the case without magnetic coupling, or lower noise level canbe achieved. Decreased amplitude of the current ripple also means thatthe RMS value of the current waveform is lower, which relates to lowerconduction losses. Lower current ripple also implies lower peaks of thecurrent, which relates to the lower stress in switching devices of thepower circuits. As the DC component of the load current is the same inboth coupled inductors (the outputs are connected to each other throughthe load so the load current is equal), and since these currents createopposite magnetic flux for arrangement shown in FIG. 14—cancellation ofthe DC component of the magnetic flux in the core is beneficial for thesmall core size and low core losses. The decrease of the current rippleis generally good for EMI decrease, and makes it easier to passregulatory requirements. While the performance of the filter in terms ofthe amplifier signals is dependent on the leakage inductance values, thenoise signals of the Common Mode (same in both output nets) will beattenuated by much larger magnetizing inductance. In this regard, CommonMode noise, often being present in circuits and representing a need foradditional high frequency filtering for the output connections, will beattenuated at much higher degree in magnetically coupled inductorarrangement in FIG. 14, as compared to the same arrangement but withoutmagnetic coupling.

The phase shifted PWM2 signal for the second differential amplifiercircuit in FIG. 12 can be created with a second PWM modulator, where theramp for the second modulator is phase shifted from the ramp for thefirst one. However, the cheaper and simpler alternative is alsoproposed, which also improves the noise immunity and insures reliablecurrent ripple cancellation, is to use one PWM modulator, and just delaythat signal by half the switching period to achieve 180 degrees phaseshift for the second channel signals, as shown in FIG. 19. As themodulator frequency is typically much higher than the maximum frequencyof the amplified signal, the introduced signal distortion can beminimized.

The magnetic components from FIG. 14 could be arranged in a singlestructure with two windings. Such structure could be called atransformer with purposely large leakage or decreased coupling.

FIG. 20 shows one possible implementation for transformer with leakage.This structure will create have leakage via air paths, but the valuewould be difficult to control accurately in a manufacturing environment.FIG. 21 and FIG. 22 show additional arrangements for transformer withleakage. FIG. 22 allows the best control of the leakage (gapvalue—spacer thickness).

The above described transducer can be a part of or contained in any typeof apparatus, including without limitation a surgical device, a cuttingtool, a fragmentation tool, an ablation tool, and an ultrasound imagingdevice.

D-Inductor and Related Circuitry

FIGS. 23A, 23B, 23C, 23D, 23E, 23F, and 23G illustrate variousembodiments of a circuit 1000 that includes a single coupled inductorpair 1100, which is herein referred to as a D-inductor. The D-inductor1100 may include a first winding L1 and a second winding L2 that aremagnetically coupled. The inductances of the windings L1, L2 may beleakage (or differential) inductances, which are described above withreference to FIG. 10. In operation, the D-inductor 1100 may storeenergy, ramp up the energy during the switch-on time, ramp down theenergy during switch-off time, and transfer the energy to the load at acontrolled rate. Also, windings L1, L2 may have less than full couplingin order to create effective series inductance.

The circuit 1000 may include one or more LC stages. For example, in FIG.23A, the circuit 1000A includes a single D-inductor 1100 with relatedfiltering capacitors. In FIG. 1B, the circuit 1000B includes twocascaded LC stages, in which the first LC stage includes a D-inductor1100A and a second filter stage includes a common mode choke 1100B. InFIG. 1C, the circuit 1000C includes an N number of cascaded LC stages,in which the first LC stage includes a D-inductor 1100A, and the secondto the Nth LC stages includes common mode chokes 1100B, . . . , 1100N,respectively.

In an embodiment of the invention, the circuit 1000 may be referenced toone or more return current planes, as shown in FIGS. 23A, 23B, and 23C.Alternatively, the circuit 1000 may not be referenced to any returncurrent plane. In an embodiment of the invention, the circuit 1000 maybe referenced to a common plane, as shown in FIGS. 23A, 23B, and 23C.Alternatively, the circuit 1000 may not be referenced to a common plane.For example, FIGS. 23D, 23E, and 23F show a single LC stage circuit1000D, an LCLC cascaded circuit 1000E, and a cascaded circuit 1000F withan N number of cascaded LC stages, in which only the first filteringstage contains the D-inductor 1100, respectively, which are notreferenced to a common plane.

Further, in an embodiment of the invention, the D-inductor 1100 may beused in combination with one or more coupled or uncoupled filteringinductors. For example, FIG. 23G shows a circuit 1000G having an Nnumber of cascaded LC stages, in which the first LC stage includes theD-inductor 1100A, the second LC stage includes an uncoupled inductors1100′A, the third and Nth LC stages include common mode chokes 1100′B,1100′N, respectively. In an embodiment of the invention, the circuit1000G may be referenced to a common connection, as shown in FIG. 3, or,alternatively, may not be referenced to any common connection. Otherconstructions are also contemplated for the circuit 1000.

The circuit 1000 may be used in applications that require or desire highswitching frequencies. For example, the circuit 1000 may be used inultrasonic applications, which typically require signals to be amplifiedwith substantially higher frequencies and involve high frequencycarriers. A high switching frequency may allow to use smaller magnetsand capacitors in output filters, but further increasing of theswitching frequency may be limited by efficiency concerns and highthermal losses. By using the circuit 1000, a switching frequency may belowered and ripple current may be reduced, which may normally beachieved only at a substantially higher switching frequency inconventional output stage designs.

The circuit 1000 may also be used in audio applications, in which anamplified signal typically has more low frequency components than inultrasonic applications. Further, in the audio applications, theswitching frequency is typically more separated from a targetedamplifier bandwidth to decrease the size of the output filters andimprove a signal-to-noise ratio (SNR) for better audio quality. Thecircuit 1000 may be used to reduce the switching frequency for higherefficiency while maintaining the inductor current ripple at a level nothigher than the conventional design. The trade-off of ripple current forhigher efficiency may maintain the SNR at the output signal at the samelabel as the conventional design, or improve the SNR, if necessary.Further, by lowering the switching frequency, the circuit 1000 maycontribute to substantial improvement in efficiency due to smallerswitching losses.

FIG. 24A illustrates a class D amplifier 2000 that includes the circuit1000, which is constructed according to an embodiment of the invention.The amplifier 200 may include two half bridges and the D-inductor 1100as part of its output filter, and may produce differential outputs suchthat the output current may be sourced from one output of the D-inductor1100 and sunk at another output of the D-inductor 1100.

The amplifier 2000 may include a plurality switches 2100, such as, e.g.,switches 2100A, 2100B, 2100C, 2100D. The switches 2100A, 2100B may beconnected in series, and the switches 2100C, 2100D may also be connectedin series. The switch pair 2100A, 2100B and the switch pair 2100C, 2100Dmay be connected in parallel between a power source and a ground. A nodeN1 between the switches 2100A, 2100B may be connected to the first coilL1 of the D-inductor 1100. A node N2 between the switches 2100C, 2100Dmay be connected to the second coil L1 of the D-inductor 1100.

A first control signal PWM1 may be used to control the switch pair2100A, 2100B, and a second control signal PWM2 may be used to controlthe switch pair 2100C, 2100D. For example, the switch 2100A may becontrolled by the first control signal PWM1, and the switch 2100B may becontrolled by an inverted signal of the first control signal PWM1. Also,the switch 2100C may be controlled by an inverted signal of the secondcontrol signal PWM2, and the switch 2100D may be controlled by thesecond control signal PWM2.

According to an embodiment of the invention, the control signal pairPWM1, PWM2 may be identical signals having different phases. The phasedifference between the first and second control signals PWM1, PWM2 maycontribute to minimizing ripple in the differential output voltagebetween output voltages V_(O1), V_(O2) of the circuit 1000, which isdescribed below in detail. The phase difference may also contribute toreducing the electromagnetic interference (EMI) by preventing the twohalf bridges from switching at the same time.

In an embodiment of the invention, the phase shift between the controlsignal pair PWM1, PWM2 may be achieved by using two related rampsignals. For example, as shown in FIG. 24B, first and second rampsignals RAMP1, RAMP2 having a phase difference (e.g., 180 degrees) maybe compared to an audio signal S_(AUDIO) to produce the control signalpair PWM1, PWM2. FIG. 24C illustrates waveforms of the first ramp signalRAMP1, the second ramp signal RAMP2, the audio signal S_(AUDIO), thefirst control signal PWM1, and the second control signal PWM2.

Other methods are also contemplated to achieve the phase shift betweenthe first and second control signals PWM1, PWM2. For example, accordingto another embodiment of the invention shown in FIG. 24D, the controlsignal PWM2 may be produced by delaying the control signal PWM1 by,e.g., a half the switching period. This approach may be more costeffective because it may not require a modulator that compares thesecond control signal PWM2 and the audio signal S_(AUDIO). This approachmay be viable when the main carrier frequency is substantially higherthan the highest frequency in the amplified signal.

In the amplifier 2000, the magnetic flux from the audio signal S_(AUDIO)may be cancelled by the D-inductor 1100. Each of the windings L1, L2 ofthe D-inductor 1100 may have twice the frequency because a pulsedvoltage from one of the windings L1, L2 may appear in the other. Thevoltage applied across each of the windings L1, L2 may be an average oftwo voltages applied across both windings L1, L2, which may lower theeffective voltage and the current ripple.

FIGS. 25A and 25B show voltage V′_(X1) and current I′_(L1) of a class Damplifier using an uncoupled inductor pair in replacement of theD-inductor 1100, which is referred to as an uncoupled design. Thevoltage V′_(X1) may be applied to one of the uncoupled inductor pair andswing between the source voltage V_(DC) (e.g., 50V) and the groundvoltage (e.g., 0V). The current I′_(L1) may be detected across theuncoupled inductor. Assuming that the voltage V_(X1) is pulsed at afrequency Fs, a period for each pulse may be calculated from dividing aduty cycle (D) by the frequency Fs. When the inductance of the uncoupledinductors is L, the current ripple in the uncoupled design may becalculated from the following equation.

${\Delta\;{I_{UNCOUPLED}(D)}} = {\frac{V\;{{dc}\left( {1 - D} \right)}}{L} \cdot \frac{D}{Fs}}$

Accordingly, when the uncoupled inductor pair is used, the currentripple may depend on the voltage V_(X1) applied thereto, the inductancevalue of the uncoupled inductors, and the switching time only.

FIGS. 26A and 26B show waveforms of voltages V_(X1), V_(X2) and currentI_(L1) of the same amplifier as FIGS. 25A, 25B except for the circuit1000 being used instead of the uncoupled inductor pair, which isreferred to as a D-inductor design. As shown in FIG. 24A, the voltageV_(X1) may be applied to the first winding L1 of the D-inductor 1100,and the voltage V_(X2) may be applied to the second winding L1 of theD-inductor 1100. Both of the voltages V_(X1), V_(X2) may swing betweenthe source voltage V_(DC) (e.g., 50V) and the ground voltage (e.g., 0V).The current I_(L1) may be detected across the first winding L1. Thecurrent ripple in the D-inductor design may be calculated from thefollowing equation.ΔI _(COUPLED)(D)=ΔI _(UNCOUPLED)(D)·RippleRatio

The ripple Ratio may be calculated from the following equation.

${RippleRatio} = \frac{\Delta\;{I_{COUPLED}(D)}}{\Delta\;{I_{UNCOUPLED}(D)}}$

Accordingly, the current ripple in the D-inductor design may depend onthe voltages V_(X1), V_(X2), the inductance values and switching timingsof both of the first and second windings L1, L2. Also, as compared tothe uncouple design, the effective switching frequency of the currentripple in the current I_(L1) may be doubled.

Considering the class D amplifier 2000 with a widely varying duty cycleD, the current ripple cancellation ratio may be derived from thefollowing equations.

${\frac{\Delta\;{I_{COUPLED}(D)}}{\Delta\;{I_{UNCOUPLED}(D)}} = \frac{\frac{1 + \rho}{1 + {2\rho}} - \left( {1 - D} \right)}{D}},{{{for}\mspace{14mu} D} < 0.5}$${\frac{\Delta\;{I_{COUPLED}(D)}}{\Delta\;{I_{UNCOUPLED}(D)}} = \frac{\frac{1 + \rho}{1 + {2\rho}} - D}{1 - D}},{{{for}\mspace{14mu} D} > 0.5}$

Here, ρ is a coupling coefficient (ρ=Lm/L_(L)), wherein Lm is amagnetizing inductance value that is related to magnetic couplingbetween the windings L1, L2, and L_(L) is the leakage inductance valueof the windings L1, L2 of the D-inductor 1100, in which the coupledwindings L1, L2 have a symmetric construction with a 1:1 turn ratio andleakages L_(L1), L_(L2) thereof are of the same value.

FIG. 27 illustrates normalized current ripple as a function of dutycycle D for coupling coefficient ρ at different values, for example,Lm/L=1, Lm/L=3, and ideal, in a test class D amplifier with theuncoupled design and the coupled design. The current ripple in theuncoupled design is largest at D=0.5, which indicates that the ratio ofthe current ripple to the amplified signal increases dramatically at alower volume setting as compared to a higher volume setting. On thecontrary, the current ripple in the D-inductor design is smallest atD=0.5, which corresponds to zero crossing or zero amplitude of thedifferential signal. The current ripple of the D-inductor design issmallest at the low signal amplitude, which corresponds to a low volumesetting in an audio application. Thus, the D-inductor design may improvethe SNR substantially in low volume settings. Furthermore, in asituation that a switching frequency is limited to a certain minimumrange, at which the SNR reaches the maximum specification, theD-inductor design may allow the switching frequency to be substantiallylowered, which may be desired for improvement in efficiency.

FIGS. 28A and 28B illustrate oscilloscope images of test results using a2×150 W stereo test amplifier with the identical testing conditions, forexample, Vdc=50 V, Fs=384 kHz, L1=L2=10 μH, C1=C2=1 μF, and load RL=8,for the uncoupled design and the D-inductor design. Measurements weretaken on a single channel, and an input sine reference signal of 1 kHzwas used. FIG. 28A shows the output waveforms at near full output powerfor the uncoupled design. FIG. 28B shows the same waveforms as FIG. 28Abut for the D-inductor design. A considerable reduction in inductorcurrent ripple is observed with the D-inductor design. The testingresult also confirms that the minimum inductor current ripple occursaround D=0.5, where the output voltage waveform crosses zero for theD-inductor design.

FIG. 29 illustrates normalized inductor current ripple as a function ofa duty cycle D for the uncoupled design and the D-inductor design atFs=384 Khz, 154 Khz in a test class D amplifier that allows adjustingthe switching timing in the output stage and changing the switchingfrequency in the wide range. The switching frequency for the D-inductordesign was decreased from 384 Khz to 153.6 Khz, which corresponds to thecase where the maximum current ripple (at duty cycle values D_(max1),D_(max2)) matches the current ripple of the uncoupled design at the sameduty cycle. The testing result indicates that the absolute currentripple maximum of the uncoupled design is about 20% higher than theD-inductor design.

FIGS. 30A and 30B illustrate inductor current ripple of a test class Damplifier with the uncoupled design, for example, 2×10 μH, operated atFs=384 Khz, and with the D-inductor design operated at Fs=154 Khz. It isobserved that the D-inductor design provides lower current ripple to theoutput at a lower switching frequency. Also, the D-inductor design hasthe minimum current ripple at zero crossings with D=0.5 while theuncoupled inductor pair has the maximum current ripple with D=0.5. Thisindicates that the D-inductor design exhibits a significantly better SNRat a low output power and a slightly better SNR at a maximum outputpower.

FIG. 31A illustrates an efficiency graph of a test class D amplifierwith the uncoupled design operated at Fs=384 Khz and the D-conductordesign operated at Fs=384 Khz and 154 Khz. FIG. 31B illustratesefficiency improvement due to the D-inductor design in the test class Damplifier. The test amplifier with the D-inductor design exhibits asubstantially higher efficiency than the uncoupled design whileproviding substantially lower current rippled to the output. Theefficiency benefit may decrease at a higher load as the conduction lossrelated to the audio signal amplitude may start to dominate. This maynot be an issue for consumer audio products because they are typicallyused at less than full power.

FIG. 32A illustrates a loss graph of a test class D amplifier with theuncoupled design at Fs=384 Khz and the D-inductor design operated atFs=384 Khz and 154 Khz. FIG. 32B illustrates a loss delta graph of atest class D amplifier with the D-inductor design. FIGS. 32A and 32Bshow that less heat is dissipated in the amplifier, which may lead tohigher component reliability and margin on a maximum ambienttemperature. Alternatively, the heat dissipation may be traded for asmaller and cheaper thermal solution.

The class D amplifier with the coupled inductors may be configured andoperated in various ways. For example, FIG. 33A illustrates amulti-phase class D amplifier 2000A, in which the two coupled-inductordesigns shown in FIG. 13 are connected in a differential way accordingto an embodiment of the invention. The multi-phase class D amplifier2000A may include a pair of phase-coupled inductors, of which theinductance value may be the product of the inductance value of thesingle inductor multiplied by the number of the inductors connected inparallel. FIG. 33B illustrates various signals, such as, e.g., audiosignal S_(AUDIO), ramp signal pair RAMP1, RAMP2, control signal pairPWM1, PMW2, for particular arrangement of the drivers and invertingdrivers in FIG. 33A. FIG. 33C illustrates the current ripple in eachinductor winding in the amplifier 2000A with the coupled inductor designshown in FIG. 33A (solid line), and the current ripple in the amplifier2000A with the uncoupled design (dotted line). FIG. 33C shows that thecurrent ripple in each inductor winding of the amplifier 2000Acorresponds to the two phase current ripple cancellation describedabove.

According to an embodiment of the invention, more phases may be added tothe class D amplifier with the D-inductor design, while creating asingle magnetic component for the whole circuit to achieve additionalbenefits. For example, FIG. 34B shows a class D amplifier 2000C that isconstructed by adding an N number of inductors to a single phase class Damplifier 2000B shown in FIG. 34A. Each phase may be added to a halfbridge for more ripple cancellation. For the same AC filter performance,i.e., same equivalent circuit, the inductance values may be the productof the inductance value of the single inductor multiplied by the numberof the inductors connected in parallel. For example, FIG. 35A shows amulti-phase class D amplifier 2000D, which includes the singleD-inductor design with four windings. FIG. 35B shows various signals,such as, e.g., audio signal S_(AUDIO) and four ramp signals RAMP1,RAMP2, RAMP3, RAMP4, that are used to generate four control signalsPWM1, PWM2, PWM3, PWM4, in the multi-phase class D amplifier 2000D shownin FIG. 35A.

FIG. 36 illustrates an implementation of the D-inductor design 2100 ofthe class D amplifier 2000D shown in FIG. 35A, according to anembodiment of the invention, in which the pins connected to theswitching nodes Vx1, Vx2, Vx3, Vx4 are shown as squares. When more thantwo windings are inversely coupled, dot notations may not be possibleanymore. In the D-inductor design 2100, the inductors L1 and L2 may beinversely coupled to each other, and the inductors L3 and L4 may also beinversely coupled to each other. The flux from the combination of theinductors L1 and L2 may be coupled in phase with the flux from thecombination of the inductors L3 and L4, which may cancel out the fluxesdue to opposite directions of the inductor currents. The core sectionsbetween the windings may provide a desired value of the leakage toachieve targeted current ripple and filtering, and the gap to the maincore from the core sections may allow to adjust the leakage value. Otherimplementations are also contemplate, for example, as shown in FIGS.42A, 43A, 44A, 45A, 46A, 46B and 47.

FIG. 37A illustrates normalized current ripple waveforms for idealcoupling (i.e., Lm/L is large) in each winding in various class Damplifier designs. The waveform 3700A indicates the current ripple inthe uncoupled design, in which each of the inductors has a value of Lo.The waveform 3700B indicates the current ripple in the D-inductor designwith two phase D-inductor with a value of Lo each. The waveform 3700Cindicates the D-inductor design with four phase D-inductors shown inFIGS. 35A, 35B (i.e., four of 2*Lo, two inductors in parallel for eachVo pin). The waveform 3700D indicates the D-inductor design with sixphase D-inductors (i.e., three inductors in parallel for each Vo pin,with a value of 3*Lo each). A small value of the coupling Lm/L may leadto larger current ripple, but a practical range (e.g., 3<Lm/L<10) maylead to a performance level close to an ideal case. A total number ofphases in the D-inductor design may be an even number, and a half of thephases may be in one group of the half bridges and the other half may bein the other group of the half bridges.

FIG. 37B illustrates total current ripple waveforms for ideal coupling(i.e., Lm/L is large) in various class D amplifier designs. The waveform3800A indicates the total current ripple of the uncoupled design. Thewaveform 3800B indicates the total current ripple of the D-inductordesign with two phase D-inductor. The waveform 3800C indicates the totalcurrent ripple of the D-inductor design with four phase D-inductor,which is shown in FIG. 35A. The waveform 3800D indicates the totalcurrent ripple of the D-inductor design with six phase D-inductor. Asmall values of coupling Lm/L may lead to a larger current ripple, but apractical range (e.g., 3<Lm/L<10) may lead to a performance level closeto an ideal case. A total number of phases in the D-inductor design maybe an even number, and a half of phases may be in one group of the halfbridges and the other half may be in the other group of the halfbridges, as for example shown in FIG. 35A for the four phase D-inductor.

FIGS. 38A, 38B and 38C illustrate various simulation results usingvarious class D amplifier designs at the same operational condition,such as, e.g., Fs=384 KHz, Vdc=50V, Signal=1 Khz. The current I_(L1) isa current per winding. FIG. 38A shows the simulation result of theuncoupled design with 10 μH inductors. FIG. 38B shows the simulationresult of the two phase D-inductor design with 2×10 μH coupledinductors. FIG. 38C shows the simulation result of the D-inductor designwith 4×20 μH coupled inductors. FIG. 38D an enlarged view of thesimulation results of FIGS. 38A, 38B and 38C. The waveform 3850A is thecurrent I_(L1) in FIG. 38A, which shows that the current ripple occursat the fundamental switching frequency Fs. The waveform 3850B is thecurrent I_(L1) in FIG. 38B, which shows that the current ripple occursat 2×Fs. The waveform 3850C is the current L_(L1) in FIG. 38C, whichshows that the current ripple occurs at 4×Fs.

Regarding the efficiency trade-off in the D-inductor design, FIGS. 39A,39B and 39C illustrate various simulation results using various class Damplifier designs at the same operational condition, such as, e.g.,Vdc=50V, Signal=1 Khz, with the exception of the switching frequency Fs.The current I_(L1) is a current per winding. FIG. 39A shows thesimulation result of the uncoupled 10 μH design at Fs=384 Khz. FIG. 39Bshows the simulation result of the D-inductor design with 2×10 μHcoupled inductors at Fs=154 Khz. FIG. 39C shows the simulation result ofthe D-inductor design with 4×20 μH coupled inductors at Fs=154 Khz.FIGS. 40A, 40B and 40C also illustrate various simulation results usingvarious class D amplifier designs at the same operational condition,such as, e.g., Vdc=50V, Signal=1 Khz, with the exception of theswitching frequency Fs. The current I_(L1) is a current per winding.FIG. 40A shows the simulation result of the uncoupled design at Fs=384Khz. FIG. 40B shows the simulation result of the D-inductor design with2×10 μH coupled inductors at Fs=154 Khz. FIG. 40C shows the simulationresult of the D-inductor design with 4×20 μH coupled inductors at Fs=75Khz.

FIGS. 41A, 41B and 41C illustrate total output current ripple in variousclass D amplifier designs at the same operational condition, such as,e.g., Vdc=50V, Signal=1 Khz, with the exception of the switchingfrequency Fs. FIG. 41A shows the simulation result of the uncoupleddesign at Fs=384 Khz. FIG. 41B shows the simulation result of theD-inductor design with 2×10 μH coupled inductors at Fs=152 Khz. FIG. 41Cshows the simulation result of the D-inductor design with 4×20 μHcoupled inductors at Fs=75 Khz. As shown in FIGS. 41B and 41C, asignificant decrease in the switching frequency Fs may allowproportionally to decrease the switching loss and to improve theefficiency while the current ripple, i.e., noise, at the output may bestill below the uncoupled design, and the filtering properties of the LCat the output may remain the same, i.e., same corner frequency fordouble poles, and the like.

FIGS. 42A, 42B, 43A, 43B, 43C, 43D, 44A, 44B, 45A, 45B, 46A, 46B and 47illustrates various implementations of the D-inductor design accordingto various embodiments of the invention. FIG. 42A illustrates aD-inductor design 2100A according to an embodiment of the invention. Asshown in FIG. 42B, the core 2110A of the D-inductor design 2100A may beconstructed with two halves. The air gap 2114A, which may be anon-magnetic spacer, or the like, may adjust the magnetizing inductance.The air gap 2116A between the windings may adjust the leakage, which maybe several times smaller than the magnetizing. The D-inductor design2100A may further include a plurality of leakage core sections 2118A. Asnoted above, more phases may be added in a similar manner for each halfbridge.

FIG. 43A illustrates another D-inductor design 2100B according toanother embodiment of the invention. FIG. 43B illustrates a top view ofthe core 2110B of the D-inductor design 2100B with a leakage coresection 2120B and an air gap 2130B that are separated from the main bodyof the core 2110B. FIG. 43C illustrates a top view of the core 2110Bwith the leakage core section 2120B and the air gap 2130B attached tothe main body of the core 2110B. FIG. 43D illustrates a side view of thecore 2110B. Referring to FIGS. 43A, 43B, 43C and 43D concurrently, theD-inductor design 2100B may include one U-core section per winding,e.g., four U-cores for four windings. In the D-inductor design 2100B,more phases may be added in the similar manner for each half bridge.Also, the leakage core section 2120B may be added with the controlledair gap 2130B for leakage inductance control. The air gap 2130B may be anon-magnetic spacer or the like.

FIG. 44A illustrates a D-inductor design 2100C according to anotherembodiment of the invention. FIG. 44B illustrates the core section 2110Cof the D-inductor design 2100C. The core section 2110C may include aleakage core section 2120C, an air gap 2130C for leakage inductancecontrol, and another air gap 2140C for magnetizing inductance control.Thus, the core section 2110C may be constructed with two pieces and theleakage core section 2120C, in which more phases may be added in thesimilar manner for each half bride. The air gap 2130C may be providedfor the leakage core section 2120C to adjust the leakage, which may beseveral times smaller than the magnetizing inductance. The air gap 2140Cunder the windings may adjust the magnetizing inductance.

FIG. 45A illustrates a D-inductor design 2100D according to anotherembodiment of the invention. FIG. 45B illustrates the core section 2110Dof the D-inductor design 2100D, in which the core section 2110D isconstructed with two pieces with an air gap 2120D therebetween formagnetizing inductance control. The core section 2110D may furtherinclude external inductors 2130D, each of which may be provided for eachwinding. The external inductors 2130 may be any off-the-shelf discreteinductor, such as, e.g., toroid, staple, power or molded core,rectangular ferrite shape, and the like. More phases may be added in thesimilar manner for each half bridge.

FIG. 46A shows a D-inductor design 2100E, in which all of the windingsare reverse coupled, i.e., out of phase, to each other, given the showndirection of the currents. The D-inductor design 2100E may be modifiedto improve the layout. For example, as shown in FIG. 46B, a half of thewindings may be mirrored such that all of the load connections are onthe same side, and all of the Vx connections are also on the same side.All of the windings may be still reverse coupled, i.e., out of phase, toeach other, given the shown direction of the currents. This arrangementmay improve the layout because the D inductor 2100F may be placedbetween the power stages and the load with output filter capacitors.FIG. 47 shows a D-inductor design 2100G according to another embodimentof the invention, which shows the layout improvement shown in FIG. 46Bapplied to the D-inductor design 2100G with an increased number ofphases, e.g., 2×3. Further, similar to FIG. 46B, a half of the windingsmay be mirrored such that the load connections are on the same side andthe Vx connections are also on the same side. All of the windings may bestill reverse coupled, i.e., out of phase, to each other.

Accordingly, the D-inductor design in class D amplifiers may allowminimization of output filters since two independent inductors arechanged into a single one where DC flux cancels out and AC flux has morethan two-fold improvement in amplitude for the two phase D-inductor, andlarger improvement for a larger number of D-inductor phases. The morethan two-fold improvement in the current ripple of the output stage maybe traded for better efficiency and thermal performance while stillmaintaining low SNR at the output. This may contribute to saving energyin general and increasing the battery life in portable devices. Theunique D-inductor property that minimum current ripple occurs at zerocrossing may offer a significantly better SNR and may open otherpossibilities for design optimization in terms of cost and performance.

Accordingly, the D-inductor design may be used in consumerelectronics/audio applications, such as, e.g., home theater system,radio receiver, car audio, cell phone, telephone speaker, laptop audio,or mobile audio, and the like. Other audio applications can includedistributed audio systems with multiple channels for buildings or parks,amplifiers for hearing aid. Also, the D-inductor design may be used todrive ultrasonic transducers and piezo devices, as noted above.

Other applications for the D-inductor design may include a DC-ACinverter for, e.g., solar or wind power, and the like, where DC voltagedirectly from the power source, such as, e.g., solar panel, windturbine, or the like, or DC voltage at the output of possibly insertedmaximum power point tracker or other power conditioning or regulatingequipment, may be modulated at a power line frequency either for directuse as an AC power source, or injecting the generated power into thepower grid.

The D-inductor design may also be used in a motor driver. For example,the D-inductor deign may be used to control a DC motor with varying DCvoltage with a desired value and polarity for the targeted speed anddirection of the motor rotation, which may be adjusted on demand.Further, the D-inductor design may be used for DC-AC applications todrive an AC motor.

While several particular forms of the invention have been illustratedand described, it will also be apparent that various modifications canbe made without departing from the scope of the invention. It is alsocontemplated that various combinations or subcombinations of thespecific features and aspects of the disclosed embodiments can becombined with or substituted for one another in order to form varyingmodes of the invention. Accordingly, it is not intended that theinvention be limited, except as by the appended claims.

We claim:
 1. A circuit comprising: a first input terminal for receivinga first pulsed signal; a second input terminal for receiving a secondpulsed signal; a load; and a plurality of LC filters connected to eachother in series, wherein one of the plurality of LC filters comprises acoupled inductor that comprises a first winding and a second windingthat are at least partially magnetically coupled to each other in amanner that both couples the first pulsed signal and the second pulsedsignal and presents differential inductance to the first pulsed signaland the second pulsed signal; and wherein the second pulsed signal is aphase-shifted signal of the first pulsed signal, and wherein thecoupling of the first pulsed signal and the second pulsed signaloperates to combine the first pulsed signal and the second pulsed signalinto one signal with less ripple than either the first pulsed signal andthe second pulsed signal have separately.
 2. The circuit of claim 1,wherein only one of the plurality of LC filters comprises the coupledinductor.
 3. The circuit of claim 1, wherein the plurality of LC filterscomprises: a first LC filter comprising the coupled inductor; and asecond LC filter comprising one of an uncoupled inductor and a commonmode choke.
 4. The circuit of claim 1, wherein the circuit is referencedto a return current plane.
 5. The circuit of claim 1, wherein thecircuit is referenced to a common plane.
 6. The circuit of claim 1,wherein a voltage applied across each of the first and second windingsis an average of the first and second pulsed signals.
 7. The circuit ofclaim 1, wherein a frequency of a first current flowing through thefirst winding is doubled by the second pulsed signal applied to thesecond winding.
 8. An output filter comprising the circuit of claim 1.9. An amplifier comprising the output filter of claim
 8. 10. Theamplifier of claim 9, wherein the amplifier comprises a class Damplifier.
 11. The amplifier of claim 10, further comprising: a firstswitch pair comprising first and second switches coupled in series andcontrolled by a first control signal to generate the first pulsedsignal; and a second switch pair comprising third and fourth switchescoupled in series and controlled by a second control signal to generatethe second pulsed signal, wherein the first and second switch pairs arecoupled in parallel between a voltage source and a ground, and whereinthe first input terminal is connected to a first node between the firstand second switches, and the second input terminal is connected to asecond node between the third and fourth switches.
 12. The amplifier ofclaim 10, wherein the first switch is controlled by the first controlsignal, and the second switch is controlled by an inverted signal of thefirst control signal, and the third switch is controlled by an invertedsignal of the second control signal, and the fourth switch is controlledby the second control signal.
 13. The amplifier of claim 11, furthercomprising: a first modulator configured to produce the first controlsignal based on a first ramp signal and an audio signal; and a secondmodulator configured to produce the second control signal based on asecond ramp signal and the audio signal, wherein the second ramp signalis a phase-shifted signal of the first ramp signal.
 14. The amplifier ofclaim 11, wherein the second control signal is produced by delaying thefirst control signal.
 15. A method of filtering a signal, said methodcomprising: providing a plurality of LC filters connected to each otherin series, wherein one of the plurality of LC filters comprises acoupled inductor that comprises a first winding and a second winding;applying a first pulsed signal to the first winding; applying a secondpulsed signal to the second winding, wherein the first and secondwindings are at least partially magnetically coupled in a manner thatboth couples the first pulsed signal and the second pulsed signal andpresents differential inductance to the first pulsed signal and thesecond pulsed signal, providing an output current to a load coupledbetween the first and second windings, the output current being sourcedfrom the first winding and sunk at the second winding, wherein thesecond pulsed signal is a phase-shifted signal of the first pulsedsignal, and wherein the coupling of the first pulsed signal and thesecond pulsed signal operates to combine the first pulsed signal and thesecond pulsed signal into one signal with less ripple than either thefirst pulsed signal and the second pulsed signal have separately. 16.The method of claim 15, further comprising generating the first andsecond pulsed signals based on first and second control signals, thesecond control signal comprising a phase-shifted signal of the firstcontrol signal.
 17. The method of claim 16, further comprisinggenerating the first and second control signals based on an input signaland first and second ramp signals, the second ramp signal comprising aphase-shifted signal of the first ramp signal.
 18. The method of claim16, wherein the first control signal is generated based on the firstramp signal and the input signal, and the second control signal isgenerated based on the second ramp signal and the input signal.